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Chapter 42.5 (Introduction)
• Gas Exchange (respiration)- the uptake of O2 from the environment and the release of CO2 into the environment
* supports the production of ATP IN cellular respiration
* involves the respiratory and circulatory system
• Gas exchange and cellular respiration are distinct processes
* Cellular respiration (the metabolic process in which an
organism obtains energy) occurs in individual cells of the
organism
* Gas exchange supports cellular respiration by constantly
supplying oxygen and removing carbon dioxide
• Respiratory Medium- the source of O2
* air for terrestrial animals
* water for most aquatic animals
• Earth's atmosphere is composed of much more oxygen than water (by a given volume)
* Bodies of water contain dissolved oxygen
- the warmer and saltier the water, the less DO
- air is less viscous than water
- acquiring oxygen from air requires 1/10 the energy than
from water
• Respiratory Surface- part of the animal's body where gases are exchanged with the surrounding environment
* Movement of O2 and CO2 across the surface occurs by
diffusion
- the rate of diffusion is proportional to the surface area on
which diffusion occurs
- inversely proportional to the square of the distance
molecules move
Respiratory Surfaces cont'd
• Diffusion cont'd
* All living cells must be bathed in water to maintain their
plasma membrane
- Respiratory surfaces of all animals are moist
- O2 and CO2 must dissolve in water before diffusing
across these surfaces
• Problem: The respiratory surface must supply O2 and expel CO2 for the entire body
• Solutions which have evolved over time:
- the size of the organism
- whether it lives in water or on land
- metabolic demands for gas exchange
• Respiratory surfaces tend to be thin and have a large surface area
- maximizes rate of gas exchange
• Gas exchange in simple animals (i.e. sponges, cnidarians, worms, unicellular organisms, etc.)
- use their entire outer skin as a respiratory organ
Example: earthworms and some amphibians have moist skin, and gas exchange occurs by diffusion across the general body surface
* These types of animals are usually small, long and thin or flat,
and with a high ratio of surface area to volume
• Large animals lack surface area to exchange gases for the whole body
• They have a respiratory organ that is excessively folded
or branched
- increases surface area
• Three most common respiratory organs:
- Gills
- Tracheae
- Lungs
• The total surface area of the gills is often much greater than that of the rest of the body
• Water has advantages
- The aqueous environment keeps the plasma membranes
of the respiratory surfaces moist
• Water has disadvantages
* O2 concentrations in water are low
Ventilation
• Ventilation- increasing the flow of the respiratory medium over the respiratory surface
* helps aquatic animals obtain enough O2 from water
* without ventilation:
- low O2, and high CO2 levels
* Ventilation in fish:
- water enters through the mouth
- passes through slits of pharynx
- flows over gills
- exits the body
• Countercurrent exchange- blood flows in the direction opposite to the movement of water past the gills
- makes it possible to transfer O2 to the blood
* Along the entire capillary, there is
a diffusion gradient favoring the
transfer of O2 from water to blood
• This exchange mechanism
- allows the gills to remove more
than 80%of the dissolved O2
passing over the respiratory
surface
- affects temperature regulation
• Advantages of air over water
* contains more oxygen (about 210 mL O2 per liter of air)
* O2 and CO2 diffuse faster in air
- less vigorous ventilation
* less energy is required
- air is lighter
- easier to pump
- less volume of air needs to be inhaled
• Disadvantages of air over water
* respiratory surface (must be large and moist) loses water to
air by evaporation
Solution: A respiratory surface folded into the body
Tracheal Systems in Insects
• Tracheal system- made up of air tubes that branch throughout the body
* tracheae- the largest tubes
- open to the outside
- finest branches extend to the surface of nearly every cell
~ gas exchange occurs at the terminal ends of the
system
* there is a very short distance between the respiratory
medium and all body cells
- the circulatory system is not involved in gas exchange
• Types of insects
* Small insects
- process of gas gas exchange if fulfilled by diffusion
through the trachea
* Larger insects
- higher energy demands
- ventilate tracheal systems with rhythmic body movements
that compress and expand the air tubes
* Insects in flight
- very high metabolic rate
- alternating contraction and relaxation of flight muscles
compress and expand the body
* Flight muscle cells are packed with mitochondria
- tracheal tubes supply these ATP generating organisms
with ample O2
Lungs
• Lungs are restricted to one location
* circulatory system fills gap between the respiratory surface
and all other parts of the body
- transports gases between lungs and the rest of the body
• Lungs have a dense net of capillaries under epithelium that forms the respiratory surface
• Lungs have evolved in animals such as:
-spiders, terrestrial snails, and vertebrates
• Some vertebrates (i.e. amphibians) have small lungs
* rely heavily on diffusion across other body surfaces
for gas exchange
Example: turtles perform gas exchange across moist epithelial
surfaces in their mouth and anus
• Most reptiles, all birds, and mammals
* rely entirely on lungs for gas exchange
• Evolutionary aspect
* Lungs and air breathing have evolved in a few aquatic
vertebrates
- adaptation to living in O2-poor water
- adaptation to spending part of their time exposed to air
Mammalian Respiratory System: A Closer Look
• Ventilation- supply of air to the lungs
* a ventilation is needed to maintain high concentration
gradients in the alveoli
• Located in the thoracic cavity
• Spongy texture
* honeycombed with moist epithelium
- functions as respiratory surface
• A system of branching ducts conveys air to the lungs
Process of Gas Exchange in Mammals
• Nasal Cavity
* air is filtered by hairs
- warmed, humidified, and sampled for odors
• Pharynx- an intersection where the paths for air and food cross
• Larynx- when food is swallowed, the larynx moves upward and tips the epiglottis over the glottis (the opening of the windpipe)
- allows food to go down the esophagus to the stomach
- opening of the epiglottis enables breathing
• Larynx
* adapted as a voice box
- sounds are produced when voluntary muscles in the
voice box are tensed
- stretching of vocal cords causes muscles to vibrate
• Trachea- windpipe
* C-shaped rings of cartilage maintain shape
• Bronchi- trachea forks into two bronchi, each leading to a lung
• Bronchioles- finer tubes within the lung which the bronchus branches in to
• Branches are covered by cilia and a thin film of mucus
- mucus traps particulate contaminants
- moving cilia move the mucus upward to the esophagus,
cleansing the respiratory system
• Alveoli- a cluster of air sacs at the ends of the tiniest bronchioles
• Gas exchange occurs across the thin epithelia of the lung's
millions of alveoli
- sufficient to carry out gas exchange for the entire body
* O2 entering the alveoli dissolves in the moist film
- diffuses across epithelium
- enters a web of capillaries surrounding each alveolus
* CO2 diffuses in the opposite direction
- from the capillaries
- across the epithelium of alveolus
- into air space